In some designs of SMPS, the switching element comprises separate but series-connected power switch and control switches. This is currently most frequently the case, and often may be necessary, in designs in which the power switch is a high-voltage bipolar junction transistor. However it should be mentioned at the outset, that a separate power switch and control switch may also be used in designs in which the power switch is a high-voltage MOSFET. Since separate control and power switches are most commonly used with bipolar junction transistors, designs utilising these two switches are often loosely referred to as emitter switched converters. Relative to high-voltage MOSFETs, high-voltage bipolar devices may be significantly cheaper, but since they suffer the drawback of low voltage amplification, they typically require a large base driver current.
Switched mode power supplies are typically operated under the control of a controller, which controls the timing of the switching of the switching element. In the case of emitter switched converters the switching element may switch the control switch, and the power switch is arranged and configured to switch following the operation of the control switch.
A schematic of a SMPS connected to an AC mains 120 through rectifier B1 and input smoothing capacitor C1, and driving LEDs is shown in FIG. 1. In this case the SMPS is a buck converter. The converter comprises a series arrangement of a control switch S1, a power switch S2, and an inductor L1. In this arrangement, the load, which is shown as a series string of LEDs is connected in series with the inductor L1. As shown, power switch S2 is a bipolar transistor which requires a relatively high base current. In order to provide this base current for the power switch S2, the base terminal of the switch S2 is connected to the smoothed rectified mains input through a resistor R1 and a parallel arrangement of a diode D3 and a resistor R4. The value of resistor R4 is chosen so as to set the required appropriate base current for the power switch S2, and diode D3 is provided so as to ensure that S2 can be switched off quickly. The SMPS comprises a controller 110 which has a supply terminal 114 and ground terminal 112. A capacitor C2 is connected across these terminals to provide a supply voltage Vcc to the controller 110. Also shown is a sense resistor R2 which is connected to the controller and used to sense the current through the inductor in order to control the switching.
As soon as Vcc rises above the start-up voltage, the controller starts switching. However, typically a switching mode power supply controller consumes more supply current in switching state than in non-switching state. For example additional supply current is needed to drive the power switch. When the current through R1 is just enough to charge C2 to the start-up voltage level (Vstart), it may not be enough to supply the SMPS during operation. The supply voltage would then drop and could reach a voltage level were the SMPS cannot work properly anymore. Typically an under-voltage-lock-out voltage (Vstop) is therefore implemented that stops the operation of the SMPS. As soon as the SMPS stops operating, the supply current of the controller drops again, and the capacitor is able to recharge to the start-up voltage level, which will then repeat the situation were the SMPS starts and stops.
To prevent this on and off operation, a take-over supply, comprising diode D2 and resistor R3 is added to provide additional current as soon as the converter starts switching and the LED voltage rises above the Vcc level. Alternatively, for flyback converters, an auxiliary winding on the primary side of the transformer may be included to provide power to the controller, once the SMPS is in operation.